how to prevent/lock a function from returning until another separate async call has resolved?

Question

I'm working on route authentication, and am storing the authed status in a context so that other React components can check if a user is logged in. The relevant part of code is:

const [loggedInUser, setLoggedInUser] = useState(null)
const authed = () => !!loggedInUser

useEffect(() => {
  async function fetchData() {
    const response = await fetch('/loggedInUser')
      .then(res => res.json())
    setLoggedInUser(response)
  }
  fetchData()
}, [])

The quick explanation of this code is, I need user data (such as id) in parts of code so I need to store the loggedInUser object. however, for simpler tasks such as checking if a user is logged in, I'm using the function authed to check if the loggedInUser variable contains an object (user is logged in) or is null.

To check if a user is logged in, I'm using passport.js and my '/loggedInUser' route looks like this:

app.get('/loggedInUser', async (req, res) => {
  if (!req.isAuthenticated())
    return null

  const { id, name } = await req.user
    .then(res => res.dataValues)

  return res.json({ id, name })
})

The problem is, code consuming this context and checking authed() is running before the useEffect() and fetch() are able to hit the GET route and use the API response to setLoggedInUser(response). so use of authed() always returns false, even if the API response later sets loggedInUser to some object value where now authed() is true. obviously this is a race condition, but I'm not sure how to address it.

Is there an elegant solution where I can 'lock' authed() from returning a value until the useEffect() has fully 'set up' the state of loggedInUser?

One (awful) solution I'm envisioning may be something like:

const [loggedInUser, setLoggedInUser] = useState(null)
const isFinishedLoading = false // <--
function authed() {
  while (!isFinishedLoading) {} // a crude lock
  return !!loggedInUser
}

useEffect(() => {
  async function fetchData() {
    const response = await fetch('/loggedInUser')
      .then(res => res.json())
    setLoggedInUser(response)
    
    isFinishedLoading = true // <--
  }
  fetchData()
}, [])

Is there a better way to 'lock' the function authed() until loading is complete?

---

Edit:

To clarify my usage of authed() for the comments, here is a trimmed down version of my App.js

export default function App() {
  return (
    <>
      <AuthProvider>
        <Router className="Router">
          <ProtectedRoute path="/" component={SubmittalTable} />
          <Login path="/login" />
        </Router>
      </AuthProvider>
    </>
  )
}

function ProtectedRoute({ component: Component, ...rest }){
  const { authed } = useContext(AuthContext)

  if (!authed()) // due to race condition, authed() is always false
    return (<Redirect from="" to="login" noThrow />)

  return (<Component {...rest} />)
}

Answer 1

I don't understand why you need authed() to be a function, and don't use isLoggedIn directly. Also I don't see where you are setting the Context value, but anyway ...

general suggestions

Generally: In React, try to think about

• "what is the state of my app at any given moment",
• and not "what should happen in which order".

In your case:

• "which page should be displayed right now, based on the state",
• instead of "redirect as soon as something happens".

The user is authorized or is not authorized to use your app at any given moment. That is a state, and you would store this state somewhere. Let's call this state isAuthorized.

different places to store state

1. You can store isAuthorized in the Context, as long as you know it is available when you need it. If the Context is not available at the moment when you want to know if the user is authorized (which seems to be the case in your app), then you can not use the Context to store isAuthorized (at least not alone).

2. You can fetch isAuthorized every time when you need it. Then isAuthorized is not available until the fetch responds. What is the state of your app right now ? It is notReady (probably). You can store the state notReady somewhere, e.g. again in the Context. (notReady will be always initially true, so you know the app is ready only if you explicitly say so.) The App might display a Spinner and do nothing else as long as it is notReady.

3. You can store isAuthorized in e.g. the browser storages (e.g. sessionStorage). They are available across page loads, so you don't have to fetch the state every time. The browser storages are supposed to be synchronous, but indeed I would treat them as being asynchronous, because things I have read about the browser storages are not inspiring confidence.

problem and solution

What you are trying to do is to store isAuthorized in the (1) Context AND (2) fetch it every time, so you have 2 states, which need to be synchronized. Anyway, you do need to fetch isAuthorized at least once in the beginning, without that, the app is not ready to be used. So you do need synchronization and a state (3) notReady (or isReady).

Synchronizing state is done with useEffect in React (or with 'dependencies'), e.g.:

useEffect(() => {
  setIsFinishedLoading( false );    // state (3) "app is ready"
  fetchData().then( response => {
    setLoggedInUser( response );    // state (2) "isAuthorized" from fetch
    setIsFinishedLoading( true );
  }).catch( error => {
    setLoggedInUser( null );
    setIsFinishedLoading( true );
  });
}, []);

useEffect(() => {
  if( isFinishedLoading ){
    setIsAuthorized( !!response );      // state (1) "isAuthorized" in local state or Context
  }
  
}, [ isFinishedLoading, response ]);

"blocking"

You probably don't need it anyway, but:

Blocking code execution in that sense is not possible in Javascript. You would instead execute the code at some other time, e.g. using Promises. This again requires thinking in a slightly different way.

You can not do:

function authed(){
  blockExecutionBasedOnCondition
  return !!loggedInUser
}

But you can do:

function authed(){
  return !!loggedInUser;
}

function executeAuthed(){
  someConditionWithPromise.then( result => {
    authed();
  });
}